698 Mr. E. Marsden and Dr. P. B. Perkins on the 



the ionization measured for any given distance, say x cm., of 

 the source to the first gauze Gt 1 really represents the integral 

 of the curve between x and x + 03 cm. In the curve reduced 

 to 15° C. and 76 cm. Hg. (fig. 5 a) this would correspond to 

 the integral between y .x and y(w+ 0*3), where y is the calcu- 

 lated reduction factor. To get the true shape of the curve, 

 therefore, rectangles of width 0*3^ were constructed at 

 various points of the curve, and the curve 6 drawn graphi- 

 cally so as to have the same integral as the rectangles over 

 their width. 



It was found that the steep part of the curve between the 

 maximum and the end was practically a straight line, and 

 the point where this straight line cuts the range axis was 

 taken as the end of the range. The ranges measured in this 

 way were found to be at 15° C. and 76 cm. Hg.: — polonium 

 3 C 77 cm., actinium C 5*40 cm., radium C 6*92 cm., and 

 thorium C 2 8*60 cm., in good agreement with the values 

 generally accepted*. To make a comparison of the curves 

 for the different products the distances were measured from 

 the ends of the ranges and the ionization expressed as 

 percentages of the maxima. In this way the curves given 



in fig. 6 were deduced. 



Fio-. 6. 



1-0 1-5 2-0 



Centimetres from End of ftflNCE. 



2-5 



It will be at once observed from an inspection of the figure 

 that the different curves are not superposable ; for instance, 



* Rutherford, ' Radioactive Substances/ p. 668. 



